1. Field of the Invention
The present invention relates to electrochemical reduction of beryllium oxide in a solid state in an electrolytic cell.
The present invention relates particularly to electrochemical reduction of beryllium oxide in a solid state to produce high purity beryllium metal in an electrolytic cell.
2. Description of Related Art
Beryllium metal has a combination of physical and mechanical properties, such as low weight, stiffness, resistance to corrosion from acids, transparency to X-rays and other electromagnetic radiation, and electrical and thermal conductivity, that make it useful for various applications in metal, alloy and oxide forms.
Beryllium metal is used principally in aerospace and defence applications. Its high stiffness, light weight, and dimensional stability within a wide temperature range make it useful in satellite and space vehicle structures, inertial guidance systems for missiles, military aircraft brakes, structural components of military aircraft, and space optical system components.
Beryllium alloys include beryllium-copper, beryllium-nickel, and beryllium-aluminium alloys, of which beryllium-copper alloys are the most important commercially. Beryllium-copper alloys are used in a wide range of applications that require electrical and thermal conductivity, high strength and hardness, good corrosion and fatigue resistance, and non-magnetic properties. Beryllium-copper strip is manufactured into springs, connectors, and switches for use in applications in automobiles, aerospace, radar, and telecommunications, factory automation, computers, and instrumentation and control systems.
Beryllium metal is extracted from beryllium oxide-containing minerals beryl (3BeOxe2x80x94Al2O3xe2x80x946SiO2) and bertrandite (4BeOxe2x80x942SiO2xe2x80x94H2O) by chemical reduction. However, energy requirements and therefore production costs for producing beryllium by conventional chemical reduction technology currently being used are high.
An object of the present invention is to provide an alternative method of extracting beryllium metal from beryllium oxides.
The present invention was made during the course of an on-going research project on the electrochemical reduction of a range of metal oxides in a solid state in an electrolytic cell that is being carried out by the applicant.
During the course of the research project the applicant carried out experimental work on a range of different metal oxides in an electrolytic cell that included a graphite crucible that formed an anode of the cell, a pool of molten CaCl2-based electrolyte in the crucible, and a cathode that included solid metal oxides. One of the metal oxides tested by the applicant is beryllium oxide.
Accordingly, the present invention provides a method of reducing beryllium oxide in a solid state in an electrolytic cell, which electrolytic cell includes an anode, a cathode formed at least in part from beryllium oxide, and a molten electrolyte, the electrolyte including cations of a metal that is capable of chemically reducing beryllium oxide, and which method includes a step of operating the cell at a potential that is above a potential at which cations of the metal that is capable of chemically reducing beryllium oxide deposit as the metal on the cathode, whereby the metal chemically reduces beryllium oxide.
The applicant does not have a clear understanding of the electrolytic cell mechanism at this stage. Nevertheless, whilst not wishing to be bound by the comments in this paragraph, the applicant offers the following comments by way of an outline of a possible cell mechanism. The experimental work carried out by the applicant produced evidence of Ca metal in the electrolyte. The applicant believes that, at least during the early stages of operation of the cell, the Ca metal was the result of electrodeposition of Ca++ cations as Ca metal on electrically conductive sections of the cathode. The experimental work was carried out using a CaCl2-based electrolyte at a cell potential below the decomposition potential of CaCl2. The applicant believes that the initial deposition of Ca metal on the cathode was due to the presence of Ca++ cations and Oxe2x88x92xe2x88x92 anions derived from CaO in the electrolyte. The decomposition potential of CaO is less than the decomposition potential of CaCl2. In this cell mechanism the cell operation is dependent at least during the early stages of cell operation on decomposition of CaO, with Ca++ cations migrating to the cathode and depositing as Ca metal and Oxe2x88x92xe2x88x92 anions migrating to the anode and forming CO and/or CO2 (in a situation in which the anode is a graphite anode). The applicant believes that the Ca metal that deposited on electrically conductive sections of the cathode was deposited predominantly as a separate phase in the early stages of cell operation and thereafter dissolved in the electrolyte and migrated to the vicinity of the beryllium oxide in the cathode and participated in chemical reduction of beryllium oxide. The applicant also believes that at later stages of the cell operation part of the Ca metal that deposited on the cathode was deposited directly on partially deoxidised beryllium oxide and thereafter participated in chemical reduction of beryllium oxide. The applicant also believes that the Oxe2x88x92xe2x88x92 anions, once extracted from the beryllium oxide, migrated to the anode and reacted with anode carbon and produced CO and/or CO2 and released electrons that facilitated electrolytic deposition of Ca metal on the cathode.
The beryllium oxide may be any suitable type.
The beryllium oxide may be any suitable form.
By way of example, the beryllium oxide may be in the form of pellets.
Preferably the metal deposited on the cathode is soluble in the electrolyte and can dissolve in the electrolyte and thereby migrate to the vicinity of the cathode metal oxide.
It is preferred that the electrolyte be a CaCl2-based electrolyte that includes CaO as one of the constituents of the electrolyte.
In such a situation it is preferred that the cell potential be above the potential at which Ca metal can deposit on the cathode, i.e. the decomposition potential of CaO.
The decomposition potential of CaO can vary over a considerable range depending on factors such as the composition of the anode, the electrolyte temperature and electrolyte composition.
In a cell containing CaO saturated CaCl2 at 1373K (1100xc2x0 C.) and a graphite anode this would require a minimum cell potential of 1.34V.
It is also preferred that the cell potential be below the potential at which Clxe2x88x92 anions can deposit on the anode and form chlorine gas, i.e. the decomposition potential of CaCl2.
In a cell containing CaO saturated CaCl2 at 1373K (1100xc2x0 C.) and a graphite anode this would require that the cell potential be less than 3.5V.
The decomposition potential of CaCl2 can vary over a considerable range depending on factors such as the composition of the anode, the electrolyte temperature and electrolyte composition.
For example, a salt containing 80% CaCl2 and 20% KCl at a temperature of 900K (657xc2x0 C.), decomposes to Ca (metal) and Cl2 (gas) above 3.4V and a salt containing 100% CaCl2 at 1373K (1100xc2x0 C.) decomposes at 3.0V.
In general terms, in a cell containing CaOxe2x80x94CaCl2 salt (not saturated) at a temperature in the range of 600-1100xc2x0 C. and a graphite anode it is preferred that the cell potential be between 1.3 and 3.5V.
The CaCl2-based electrolyte may be a commercially available source of CaCl2, such as calcium chloride dihydrate, that partially decomposes on heating and produces CaO or otherwise includes CaO.
Alternatively, or in addition, the CaCl2-based electrolyte may include CaCl2 and CaO that are added separately or pre-mixed to form the electrolyte.
It is preferred that the anode be graphite or an inert anode.
The applicant found in the experimental work that there were relatively significant amounts of carbon transferred from the graphite anode to the electrolyte and to a lesser extent, to the beryllium produced at the cathode under a wide range of cell operating conditions. Carbon in the beryllium is an undesirable contaminant. In addition, carbon transfer was partially responsible for low energy efficiency of the cell. Both problems could present significant barriers to commercialisation of electrolytic reduction technology.
The applicant also found that the dominant mechanism of carbon transfer is electrochemical rather than erosion and that one way of minimising carbon transfer and therefore contamination of beryllium produced at the cathode by electrochemical reduction of beryllium oxide is to position a membrane that is permeable to oxygen anions and is impermeable to carbon in ionic and non-ionic forms between the cathode and the anode and thereby prevent migration of carbon to the cathode.
Accordingly, in order to minimise contamination of beryllium produced at the cathode resulting from carbon transfer, it is preferred that the electrolytic cell includes a membrane that is permeable to oxygen anions and is impermeable to carbon in ionic and non-ionic forms positioned between the cathode and the anode to thereby prevent migration of carbon to the cathode.
The membrane may be formed from any suitable material.
Preferably the membrane is formed from a solid electrolyte.
One solid electrolyte tested by the applicant is yttria stabilised zirconia.
According to the present invention there is also provided an electrolytic cell as described above and operating in accordance with the above described method.